Wireless devices or mobile stations such as cellular handsets and other wireless systems transmit and receive representations of speech waveforms. A physical layer of a cellular handset typically includes circuitry for performing two major functions, namely encoding and decoding. This circuitry includes a channel codec for performing channel encoding and decoding functions and a vocoder for performing voice encoding and decoding functions. The vocoder performs source encoding and decoding on speech waveforms. Source coding removes redundancy from the waveform and reduces the bandwidth (or equivalently the bit-rate) in order to transmit the waveform in real-time. The channel codec increases redundancy in the transmitted signal to enhance the robustness of the transmitted signal. Synchronizing these two functions allows the system to operate properly.
A number of different wireless protocols exist. One common protocol is referred to as global system for mobile communications (GSM). In a GSM system, the vocoder operates on blocks of speech data that are 20 milliseconds (ms) in duration. The channel codec transmits and receives data every 4.615 ms. Since the speech encoder (i.e., vocoder) serves as a data source to the channel encoder/modulator (i.e., channel codec) and the speech decoder (i.e., vocoder) serves as the data sink for the channel demodulator/decoder (i.e., channel codec), the vocoder and channel codec should be maintained in synchronization.
Further, the speech encoder should deliver data to the channel encoder with sufficient time margin to complete channel encoding and modulation operations before the time at which the data are transmitted over the air. Further complicating the issue are limits on the round-trip delay of the overall communications link. Hence, the vocoder cannot deliver the data too early lest the delay budget (such as that set forth by the European Telecommunications Standards Institute (ETSI)) be violated, and cannot deliver data too late lest the data be discarded. As a practical matter, the later the vocoder delivers data to the channel codec, the harder a digital signal processor (DSP) must work to complete all signal processing on schedule, thus creating a greater system load.
Adaptive multi-rate (AMR) vocoders have been introduced recently in certain cellular communication standards, such as GSM and WCDMA. AMR vocoders support multiple source rates, and compared to other vocoders, provide some technical advantages. These advantages include more effective discontinuous transmission (DTX) because of an in-band signaling mechanism, which allows for powering down a transmitter when a user of a cellular phone is not speaking. In such manner, prolonged battery life and reduced average bit rate, leading to increased network capacity is provided. AMR also allows for error concealment.
In a system supporting AMR, the bit rate of network communications can be controlled by the radio access network depending upon air interface loading and the quality of speech conditions. To handle such different bit rates, the network will send configuration messages to a cellular phone to control its transmission at a selected bit rate. During an AMR voice call, the network may send a message to the mobile station to change the AMR configuration (e.g., source rate). Since both the channel codec and vocoder use this information, careful synchronization is needed between the codec and vocoder during AMR configuration changes.
Accordingly, methods and apparatus to maintain synchronization between channel codec and vocoder would improve performance of a mobile station.